With the recent demonstration by this laboratory and others (i) that the amyloid beta protein precursor (betaAPP) is constitutively processed by betaAPP secretase to produce identifiable derivatives, (ii) that the endosomal-lysosomal system processes identifiable carboxyl-terminal betaAPP derivatives likely to be important intermediates in the pathway that produces amyloid, and (iii) that 4 kD amyloid beta protein (Abeta) is normally produced and released by cultured cells, it is now realistic to undertake the identification of the key proteases that determine the amount of Abeta produced. Here, we propose experimentation focused on the identification of betaAPP secretase that will explore the feasibility of rapidly identifying these proteases using antisense cDNA expression libraries. We plan to focus on betaAPP secretase in these experiments because it is likely to be difficult to isolate this unusual and critically important enzyme conventionally using in vitro assays based on sequence specificity and because the knockout of betaAPP secretase with antisense cDNAs will almost certainly increase the level of surface betaAPP to a degree that can be detected by fluorescence activated cell sorting.